The present invention relates to a rolling bearing suitable for use as a bearing for, for example, a swing arm which is provided in a magnetic disk apparatus and swings minutely at a high speed.
A swing arm for a magnetic disk apparatus is required to swing minutely at a high speed in order that a magnetic head disposed at the leading end of the swing arm is able to seek a target track on a magnetic disk at a high speed and with high accuracy. To realize such high-speed and minute swing motion, there is used a rolling bearing in the base end swing portion of the swing arm. FIG. 9 is a section view of a swing arm base end which is supported by a conventional rolling bearing. On the inside portion of a casing 1 for an actuator (for example, a voice coil motor) which carries out a high-speed and minute swinging motion, there is threadedly mounted a base plate 3; and, the lower end of a support shaft 5 is fitted with and fixed to one end of the base plate 3. Also, the upper end of the support shaft 5 is threadedly mounted on the casing 1 by a screw 6 which extends through the casing 1.
On the outer periphery of the support shaft 5, there are disposed a pair of ball bearings (rolling bearings) 7 in such a manner that they are spaced apart from each other in the axial direction of the support shaft 5. Each of the ball bearing 7 is disposed in such a manner that its inner race is fixed to the support shaft 5 and its outer race is fixed to a housing 9. To the outer periphery of the housing 9, there is fixed the base end of a swing arm 11, while the swing arm 11 is swingably supported on the support shaft 5 through the ball bearings 7. At the leading end of the swing arm 11, there is disposed a magnetic head (not shown). Also, on the outer periphery of the housing 9, there is disposed a voice coil portion 13, while the voice coil portion 13 is situated in a magnetic field which is formed by two permanent magnets 15 disposed within the housing 9. Accordingly, in case where a given current is caused to flow in the voice coil portion 13 by a magnetic circuit, there is applied a rotational force to thereby swing the swing arm 11 by a given amount.
By the way, into the ball bearing 7 of this type, there is enclosed lubricant such as lubricating oil or grease for the purpose of lubrication. Conventionally, the amount of enclosure of the lubricant into the ball bearing is generally set in the range of 10 to 20% of the capacity of a space formed in the inside portion of the ball bearing.
Also, conventionally, the two ball bearings 7, 7, into which grease is enclosed as lubricant, are respectively given a pre-load (that is, an axial load applied to a bearing to thereby adjust a clearance between a rolling element and inner and outer races in order to prevent the shaft of the bearing from swinging as well as prevent the rolling element from slipping, swinging and generating noises) before they are put into practical use. And, as shown in FIG. 10, adhesives are applied to the peripheries of inner race inside diameter surfaces 7a, 7a and outer race outside diameter surface 7b, 7b, so that the two ball bearings 7, 7 can be bonded and fixed to the support shaft 5 and housing 9. Also, the both side surfaces of each of the ball bearings 7, 7 can be normally sealed through seal members 73. By the way, conventionally, the pre-load is generally set in the range of 300 gf to 800 gf.
In recent years, as the magnetic disk apparatus becomes highly dense, the magnetic disk apparatus is required that a spacing between the magnetic head and magnetic disk is as narrow as possible. Due to this, for the reliability of the magnetic disk apparatus, control of contamination becomes more and more important. Especially, since the swing arm is disposed adjacent to the magnetic disk surface, the swing arm is required that it is as free as possible from the contamination. For this reason, for the ball bearing, there are needed lubricant and lubricating method which can reduce the evaporation and scattering of the lubricant further than the conventional lubricant and lubricating method.
On the other hand, the track width of the magnetic disk has been made narrower and narrower, which requires the swing arm to speed up its access to the target track and enhance the accuracy of its positioning operation. This in turn requires the ball bearing, which supports the swing arm, that it can prevent an increase in torque as well as a sudden variation in the torque (torque spike).
However, the conventional ball bearing for a swing arm generally aims at extending its life and enclosing thereinto a large amount of grease in the range of 10 to 20% of the capacity of the space formed in the inside portion of the ball bearing. Therefore, in the conventional ball bearing, there are necessary various devices to eliminate a fear that outgases can be generated due to the evaporated or scattered grease, and a fear that such outgases can stick to the surface of the magnetic disk. Also, in case where the grease enclosure amount is increased for enhancement of the durability of the bearing, the viscosity and mixing resistance of the grease increase to thereby cause the torque to vary and increase.